The major objective of this research is to firmly establish the role of PDE4B as a novel target for therapeutic exploration in diffuse large B-cell lymphoma (DLBCL), the most common B-lymphoid malignancy in adults. DLBCL is an aggressive and heterogeneous disorder in which only approximately 40% of the patients can be cured with standard therapy. More aggressive and/or dose-intensified chemotherapy regimens do not provide major survival advantages may have higher toxicity. Therefore, to improve the cure rate in DLBCL, we need to better understand its molecular basis and translate these findings into therapies that target the specific genetic lesions of these tumors. Towards this goal, we used expression profiling on microarrays and found single genes and pathways that were associated with the disease outcome. In our pilot study, one of the most prominently over expressed genes in fatal/refractory DLBCL was the phosphodiesterase 4B (PDE4B). This enzyme inactivates cAMP, a second messenger which induces cell cycle arrest and apoptosis in B-lymphocytes. Because PDE4B terminates cAMP activity, it abrogates these negative effects. Our preliminary studies confirmed that in DLBCLs PDE4B blocks cAMP-mediated apoptosis, via a unique cross-talk with the PI3K/AKT survival pathway. In this initial study, we also confirmed, in vitro, the potential of PDE4 inhibitors in the treatment of these malignancies. In the present proposal, we will build on these preliminary data to address two specific aims: 1. Define the mechanisms by which cAMP-PDE4B modulate PI3K/AKI activity in DLBCL. 2. Develop murine DLBCL models to confirm, in vivo, the effectiveness of PDE4 inhibitors in the treatment of this disease. Our study plan exploits a series of tools that we have recently generated, including a DLBCL cell line with low endogenous PDE4B activity reconstituted with a retrovirus vector to express wild-type PDE4B or a phosphodiesterase inactive mutant. In specific aim one, we will utilize these cells to define how intracellular cAMP/PDE4B modulates PI3K activity; we will specifically study cAMP effects on protein tyrosine kinases (PTK) that regulate PI3K and on the small GTPase Ras. To precisely evaluate the role of the PTKs in this process, we will also use DT-40 knockout lymphoma cells lacking relevant upstream kinases. To resolve the role of Ras, we will retrovirally generate DLBCL cells expressing constitutive active (V12) or dominant negative (N17) forms of this protein. In aim two we will generate human DLBCL xenografts in NOD/SCID mice to define the effectiveness of PDE4B inhibitors in the treatment of these tumors. Since clinical grade PDE4 inhibitors are available and currently in clinical trials for a variety of conditions, the findings derived from this proposal can realistically lead to novel therapeutic strategies in DLBCL.